As Bryanj said, check the 3 yellow wire connector coming from the alternator. The current that goes through this connector is quite high and ANY corrosion on the terminals causes a resistance and then the connector gets hot. This heating of the terminals causes the metal connector (female part) to loose their temper and the terminal becomes soft which further reduces the current carrying ability of the connector. With vibration and dirt this can cause the connection to open which can then drain the battery since the full alternator output is no longer available to charge the battery.
If this connector is in good shape i.e. each connection has a good "bite" on its spade lug and the terminals are bright and clean then I would start checking your wiring harness for shorts, worn insulation etc. I would also check the rectifier with an Ohm meter to be sure you don't have a bad diode.
-P.